1. Technology Field
The present invention relates to particles with special structure for preventing electrostatic discharge and paste containing the same.
2. Description of the Related Art
As sizes of electronic devices become smaller, densities of parts mounted therein increase and mounting integrated circuits has been increasing to improve performance and decrease volumes. As a result, there is a large demand for prevention of electrostatic discharge which is generated from electronic devices.
In response to increases in use of mobile products such as smart phones and tablet PCs, use of auxiliary equipment such as USBs, direct cables and the like is also increasing rapidly for data communications between portable devices or between a portable device and a non-portable device such as a personal computer. Surge generation from power stages and signal-connecting parts included therein also increases.
Previously, components such as varistors were used to resolve this problem. However, such varistors have disadvantages such as high capacitance. When components for preventing electrostatic discharge which have high capacitance such as varistors are used, normal data transmissions can be difficult due to generation of noises and signal distortions in recent environments having increases in data transmission amount and rate. Accordingly, there is a demand for developing new products having not only low capacitance but also excellent electrostatic discharge prevention properties for high speed data transmissions.
Electrostatic discharge (ESD) is one of inevitable phenomena caused by use of electronic systems and integrated circuits. When electrostatic discharge is caused, a peak current of several amperes is generated within several hundreds of nanoseconds and it should be conducted to a protection circuit such as GND within nanoseconds. If not, high currents move to integrated circuits to cause deteriorations and damages. Since integrated circuits are very fine with having pitches of from several tens to several hundreds of meters, when surge is generated, it can cause serious damages and unrecoverable deterioration. Therefore, developments of components for preventing electrostatic discharge are highly demanded.
There are four types of protection elements to protect circuits in components.
The first type is a laminated chip varistor which comprises a metal oxide having semiconductor properties such as ZnO and sintered body electrodes. The varistor has a function to conduct significantly increased current when an applied voltage reaches a certain definite value or more. However, it requires a complicated manufacturing process including sheet molding, electrode printing, lamination, sintering and the like and is unsuitable for high speed data transmissions due to high capacitance.
The second type is a discharge-type element having electrostatic discharging properties which can be controlled by the distance of a discharging gap, the pressure and the kind of a sealed gas and the like. It has excellent electrostatic discharge properties. However, its constitution is complicated and its size is limited as a small sized surface mounting element. Even though its operation is simple, its durability is poor.
The third type is a method of forming a discharge gap and regulating a discharge voltage by the distance of the discharge gap. Here, the moisture and gases in the environment cause contamination on conductor surface and thereby the discharge voltage can be changed or the carbonization of a substrate can cause short circuit on the electrodes during being discharged.
The fourth type is a method of coating a discharge gap with paste for preventing electrostatic discharge which is a mixture of metal or metal oxide filler and resin in order to resolve the problems mentioned above and improve insulation properties at a low voltage. The metal oxide filler, which is used in manufacturing such pastes for preventing electrostatic discharge, should have properties which ZnO has. However, when the metal is used as a filler, it requires to form an oxide film on the metal surface to keep insulation properties until reaching a certain voltage.
In properties of the paste for preventing electrostatic discharge, voltage in which resistance starts to rapidly decrease in an insulation condition is called as turn-on-voltage which is an important term to explain properties of preventing electrostatic discharge. It is preferred to have a low turn-on-voltage to show excellent electrostatic discharge prevention properties so that it is more advantageous to use a filler with a larger diameter. When a spherical-shaped filler is used, the thickness of a layer of preventing electrostatic discharge is increased with increases in the diameter of the filler which can be disadvantageous in decreasing and downsizing the size of elements. There is another drawback of increasing the effects of filler to shorten the distance between fillers to lower the turn-on-voltage.